1. Field of the Invention
The present invention relates to an optical add-drop multiplexer, and optical network equipment using the same, and more particularly to transmission of optical information by use of optical fibers.
2. Description of Related Art
The wavelength division multiplexed (WDM) optical transmission method is a very effective method for increasing the capacity of optical fiber communications. In this method, a plurality of optical signals whose wavelengths differ from one another are multiplexed into one optical fiber to transmit information. An optical add-drop multiplexer (OADM) is a kind of transmission equipment placed at each node in an optical fiber network that carries a wavelength division multiplexed signal. The optical wavelength add-drop multiplexer uses a technology by which from a WDM signal that is transmitted through an optical fiber, only an optical signal having a necessary wavelength is branched (dropped) to receive the optical signal, and an optical signal to be transmitted from this node is inserted (added) into the WDM signal. In the optical wavelength add-drop multiplexer, most WDM signals transmitted through an optical fiber can be passed through as light without converting the WDM signals into electric signals. The optical wavelength add-drop multiplexer, therefore, has an advantage of being able to reduce, to a large extent, the number of optical transceivers required for each node. Above all, the reconfigurable optical wavelength add-drop multiplexer (ROADM) uses an optical switch, or the like, and thereby can change, if necessary, a wavelength that is added/dropped at each node. The reconfigurable optical wavelength add-drop multiplexer attracts a great deal of attention because the configuration of transmission equipment is flexibly changed in response to the future growth of a network to provide wavelength paths. Its typical configuration is described in the catalogue titled “Configurable OADM Module” by Suntech, Mar. 2003, page 1. It is to be noted that hereinafter this equipment is referred to as “optical add-drop multiplexer” for the sake of simplicity.
FIG. 5 is a diagram illustrating the configuration of a typical conventional optical add-drop multiplexer 140. A wavelength division multiplexed signal, which is transmitted from other wavelength division multiplexed optical transmission equipment such as an optical add-drop multiplexer, is inputted into an optical add-drop multiplexer 140 through an input WDM optical fiber line 101. This is an example in which optical signals corresponding to eight wavelengths are wavelength division multiplexed. An optical wavelength demultiplexer 102 demultiplexes the wavelength division multiplexed signal into different through signal paths 104-1 through 104-8 that correspond to wavelengths λ1 through λ8 respectively. After that, the demultiplexed signals are wavelength division multiplexed by an optical wavelength multiplexer 110 again, and this wavelength division multiplexed signal is then transmitted from an output WDM optical fiber line 111 to other optical transmission equipment. Usually, an AWG (arrayed waveguide grating) device, a device in which a tandem connection between dielectric multi-layer film and an optical fiber grating is made, and the like, are used as the optical wavelength demultiplexer 102 and the optical wavelength multiplexer 110.
2×2 optical switches 141-1 through 141-8 used for switching to an add-drop state of an optical signal are located in the middle of the through signal paths 104-1 through 104-8 respectively. Each of the 2×2 optical switches can make switching between two states, a through state and the add-drop state. The through state is a state in which, as shown in the example of the 2×2 optical switch 141-1, an optical signal corresponding to each wavelength (for example, λ1), which is inputted from the input WDM optical fiber line 101 into each through signal path through the optical wavelength demultiplexer 102, is output to the output WDM optical fiber line 111 through the optical wavelength multiplexer 110 as it is. The add-drop state is a state in which, as shown in the example of the 2×2 optical switch 141-2, an optical signal corresponding to each wavelength (for example, λ2), which is inputted from the input WDM optical fiber line 101 into each through signal path through the optical wavelength demultiplexer 102, is dropped into the drop signal output optical fiber 108-2, and at the same time an optical signal corresponding to the wavelength λ2 inputted from the add signal input optical fiber 109-2 is connected to the through signal path 104-2, and is then output to the output WDM optical fiber line 111 through the optical wavelength multiplexer 110. To be more specific, if the optical switch 141-N is in the through state, an optical signal corresponding to the wavelength λN inputted from the input WDM optical fiber line 101 is passed through the optical add-drop multiplexer (“through”). On the other hand, if the optical switch 141-N is in the add-drop state, the optical signal corresponding to the wavelength λN is branched and extracted by the optical add-drop multiplexer (“drop”), and at the same time a new optical signal is added (“add”), and is then output to the output WDM optical fiber line 111.
Incidentally, one of the important functions of a wavelength division multiplexed optical network using OADM, and the like, is an optical protection function. This technique is disclosed in, for example, Japanese Patent Laid-Open No. Hei 6-244796. FIG. 6 is a diagram illustrating the configuration example of the conventional optical protection. This example relates to an optical 1+1 method in which a path of an optical signal having the wavelength λ1 is duplexed with active and backup signal lines, the path being provided between the node 124-1 and the node 124-2 in the ring network. In the event that a failure occurs in the active signal line, switching to the backup side is made on the receiving side. The optical network has a configuration in which three nodes 124-1 through 124-3 are connected to one another by use of two optical fiber lines, a clockwise active WDM optical fiber line 120 and a counterclockwise backup WDM optical fiber line 121. The conventional active-side optical add-drop multiplexers 143-1 through 143-3 and the conventional backup-side optical add-drop multiplexers 144-1 through 144-3 are located in the nodes 124-1 through 124-3 respectively. Here, the conventional active-side optical add-drop multiplexers 143-1 through 143-3 are connected to the active WDM signal line 120; and the conventional backup-side optical add-drop multiplexers 144-1 through 144-3 are connected to the backup wavelength path 121. The optical add-drop multiplexers in the nodes 124-1, 124-2 communicate with each other by adding or dropping a signal having the wavelength λ1.
If the optical protection is not used, an optical signal is transmitted by use of only the active WDM optical fiber line 120 without using the backup signal line 121, a 1:2 optical coupler 128, and a 2×1 optical switch 142 shown in the figure. To be more specific, the optical transmitter 125-1 is connected to the add signal input optical fiber 109-1-1 of the active-side optical add-drop multiplexer 143-1, whereas the optical receiver 126-2 is connected to the drop signal output optical fiber 108-2-1 of the active-side optical add-drop multiplexer 143-2. An optical signal is passed through the path 122-1 of an active optical signal having the wavelength λ1, and is transmitted clockwise from the node 124-1 to the node 124-2 before the optical signal is received there. At the same time, the optical transmitter 125-2 is connected to the add signal input optical fiber 109-2-1 of the active-side optical add-drop multiplexer 143-2, whereas the optical receiver 126-1 is connected to the drop signal output optical fiber 108-2-1. An optical signal is passed through the path 122-2 of an active optical signal having the wavelength λ1, and is transmitted clockwise from the node 124-2 to the node 124-1 through the node 143-3 in a through state, before the optical signal is received at the node 124-1. However, in the above conditions, if a failure 132 such as fiber cut occurs in the optical signal path, it becomes impossible to use the path 122-1 of the active optical signal. As a result, communications from the node 124-1 to the node 124-2 are disabled.
The optical protection is a function of quickly recovering a signal line when such a failure occurs. In this conventional example, the backup WDM signal line 121 through which an optical signal passes in a reverse direction is prepared. In the transmission-side node (for example, 124-1), an optical signal which is output from the optical transmitter 125-1 is branched into two by the 1:2 optical coupler 127-1. The branched optical signals are inputted into both the active-side add signal input optical fiber 109-1-1 and the add signal input optical fiber 109-1-2 of the backup-side optical add-drop multiplexer 144-1 respectively. In the backup-side WDM optical fiber line 121, this optical signal is transmitted counterclockwise along the path 123-1 of a backup optical signal having the wavelength λ1, and is passed through the node 124-3, and then arrives at the node 124-2 where the optical signal is output from the drop signal output optical fiber 108-2-2 of the backup-side optical add-drop multiplexer 144-2. A 2×1 optical switch 142-2 is located immediately before the optical receiver 126-2. The 2×1 optical switch 142-2 selects either an optical signal of the active-side drop signal output optical fiber 108-2-1 or that of the backup-side drop signal output optical fiber 108-2-2, and then inputs the selected optical signal into the optical receiver 126-2. The 2×1 optical switch 142-2 usually selects the active-side optical signal. However, if a failure 132 occurs and thereby the active-side optical signal 122-1 is interrupted, the optical switch control circuit 145-2 usually quickly switches the 2×1 optical switch 142-2 to the backup signal line side within several tens of milliseconds in response to a failure signal 107-2, and then inputs into the optical receiver 126-2 the optical signal passing through the path 123-1 of the backup optical signal so that the optical signal is not interrupted. Also for the signal line through which an optical signal passes from the node 124-2 to the node 124-1, the optical protection function is implemented completely in the same manner.